1. Field of the Invention
The present invention relates to X-ray tubes generally and, more particularly, but not by way of limitation, to a novel X-ray tube having a spherical anode.
2. Background Art
Originally, an X-ray tube had a fixed tungsten target, or anode, that produced X-rays at an angle out of the tube the X-rays being were emitted when electrons impinged on the tungsten anode. This arrangement created a problem with heating of the fixed tungsten target. The life of an X-ray tube is generally proportional to the available surface area of the target, with small target areas resulting in lower life and concomitant higher overcall costs for an X-ray installation.
In a further development, a rotating disk is used to produce a greater target area and to reduce problems with heating and heat dissipation. In yet a further development, X-ray tubes with rotating cone-shaped targets were developed. Such X-ray tubes greatly increase surface area, but still suffer from problems with limited surface area.
The increased surface area does allow the heat and energy load to be distributed over a larger surface area (focal track), thus increasing tube capacity. Additionally, the larger mass serves as a better heat sink compared with a stationary anode. Rotor speeds are usually in the range of from about 3000 to about 10,000 rpm.
The rotating shaft of such X-ray tubes is bearing mounted and usually constructed of molybdenum, a poor conductor of heat. Nevertheless, the heat from the anode is transferred to the rotor bearings and they are often the cause of X-ray tube failure.
Another limitation with conventional X-ray tubes is that there is a pronounced xe2x80x9cheel effectxe2x80x9d that reduces the intensity of the X-ray beam on the anode side of the tube. Such effect causes the X-rays produced in the area of the target to be attenuated, or partially absorbed, by the target itself before being able to be used for diagnostic purposes. This makes for a variable image, darker on the cathode side of the film.
The relatively small anode surface areas of conventional X-ray tubes limits the degree of heat removal therefrom and concomitantly limits the rate of use of such X-ray tubes, in order to prevent damage to the X-ray tubes due to overheating. Radiologists and radiologic technicians must go to some effort to make sure that an X-ray tube is not damaged by overheating. For example, radiologists and radiologic technicians have to analyze X-ray tube heating and cooling charts to determine X-ray tube output limits and cooling times before it is safe to use a tube again and must adhere to the results of the analysis to avoid overheating of the X-ray tube.
In summary, some of the limitations of conventional X-ray tubes are:
Heat loading limits tube output.
Heat build-up requires cooling time before next use.
Heat loading damages targets and shortens tube life.
Heat loading is transferred to bearings, shortening tube life.
Molybdenum stem (in rotating disk tubes) may warp, causing wobbling of the anode.
There is a pronounced heel effect.
Some devices that address increasing target area and the cleaning thereof are disclosed in the following patents:
U.S. Pat. No. 3,334,228, issued Aug. 1, 1967, to Mattson, and titled X-RAY SPECTROMETER HAVING AN X-RAY SOURCE WITH A CONTINUOUSLY CLEANED X-RAY TARGET, describes an X-ray source having a rotating cylindrical target. Cleaning of the target is effected by sputtering off contaminants with ionized gas.
U.S. Pat. No. 4,107,563, issued Aug. 15, 1978, to Oddell, and titled X-RAY GENERATING TUBES, describes in one embodiment a rotating cone-shaped target that is reciprocated along its axis of rotation to vary the path of the X-rays. In another embodiment, the surface of the target is shaped so that the path varies as the target is rotated without having to reciprocate the target.
U.S. Pat. No. 4,399,551, issued Aug. 16, 1983, to Grady, and titled X-RAY TUBE HAVING ROTATABLE TRANSVERSELY OSCILLATORY ANODE, describes a rotating disk-shaped target that is movable transversely with respect to its axis of rotation to provide a spirally arranged focal track on the surface of the target to facilitate heat dissipation.
U.S. Pat. No. 4,413,356, issued Nov. 1, 1983, to Hartl, and titled FLAT ROTARY-ANODE X-RAY TUBE, describes a rotating disk that has a cone-shaped target surface on the outer periphery thereof. The patent is directed to the bearing arrangement for the disk.
U.S. Pat. No. 4,799,249, issued Jan. 17, 1989, to Paulikas, and titled SELF-CLEANING ROTATING ANODE X-RAY SOURCE, describes a rotating cylindrical target including a self-cleaning feature similar to the 1228 patent above.
None of the foregoing patents discloses an X-ray tube having a target with a greatly increased available target area and reduced heel effect.
Accordingly, it is a principal object of the present invention to provide an X-ray tube having a target with a greatly increased area.
It is a further object of the invention to provide such an X-ray tube that facilitates cooling of the target.
It is another object of the invention to provide such an X-ray tube that reduces the heel effect.
It is an additional object of the invention to provide such an X-ray tube that is economical.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.
The present invention achieves the above objects, among others, by providing, in a preferred embodiment, an X-ray producing device, comprising: an X-ray tube; a cathode disposed in said X-ray tube to produce a stream of electrons; and a spherical anode disposed in said X-ray tube to have impinged on a point thereon said stream of electrons and to produce thereby a stream of X-rays.